A soft switching DC-DC converter for converting a voltage of an input power into a desired voltage to output can be highly improved in its efficiency by reducing switching loss with soft switching technology. Accordingly, it makes it possible to downsize the passive components such as a choke coil and a capacitor by making the drive frequency of a switching element higher. Thus, as an example of a conventional soft switching DC-DC converter, there is a soft switching DC-DC converter disclosed in Japanese Patent Laid-open No. 2004-129393.
This Patent Document 1 includes an auxiliary resonant circuit in which two serially-connected main switches and a resonant reactor and an auxiliary switch are serially connected to each other. The main switches are alternately turned on/off, and when a first switch which is either of the two main switches is on, electric energy from a DC power source connected to a terminal in the input side is accumulated in a smoothing reactor, and a second switch which is the other of the two main switches is on, the electric energy accumulated in the smoothing reactor is discharged to a load connected to a terminal in the output side. Further, both or either of the main switches include(s) the capacitance(s) to supply the electric energy to the resonant reactor from the output side when the auxiliary switch is on, thereby to use the electric energy for the resonant action of the capacitance(s) and the resonant reactor.